C3H/StWi mice were highly susceptible (45% in 8.9 mos.) to mammary carcinogenesis by DMBA. All preneoplastic lesions were ductal hyperplasias, rather than hyperplastic alveolar nodules as in MMTV-carcinogenesis and the tumors were largely ductal papillomas. Virus-specific RNA levels were not increased over background in the tumors. Immunologic studies thus far have indicated the absence of virus-specific antigens in the tumors. The cocarcinogenic effect of the presence of exogenous MMTV is being evaluated in MMTV-infected C3H/StWi mice. Hybrids between C3H/Avy fB (90% mammary cancer) and C3H/StWi (1% mammary cancer) and backcross progeny, produced by mating C3H/StWi males with F females, show significantly reduced mammary cancer incidence, 28 and 8% respectively. Hybirdization analysis for MMTV RNA in lactating mammary tissue from each parent strain, the F1 offspring, and the BC segregants demonstrated MMTV RNA levels to be 2-3 fold greater in the C3H/St/Wi glands than in C3H/Avy fB; however, no MMTV-specific translation products were detected in C3H3/StWi glands. Transcription levels in F1 and BC1 females were indistinguishable; however, the presence of immunologically-detectable MMTV structural proteins seemed to segregate with the Avy gene.